class_docstr: Smooth L1 loss
  Args:
      sigma (float): hyper param in smooth l1 loss
code: def __init__(self, sigma=1.0):
      super(SmoothL1Loss, self).__init__()
      self.sigma = sigma
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from collections import OrderedDict
  from paddle import fluid
  from paddle.fluid.param_attr import ParamAttr
  from paddle.fluid.initializer import Normal, Xavier
  from paddle.fluid.regularizer import L2Decay
  from paddle.fluid.initializer import MSRA
label_desc: Use image node_id 1 to create a new SmoothL1Loss object from inherited base classes: object with example: obj = SmoothL1Loss(sigma)